Abstract CPT-11 is a highly effective, camptothecin-based anticancer agent that is currently approved for the treatment of colon cancer. This compound is a prodrug and is activated by carboxylesterases to yield SN-38, a potent topoisomerase I poison. The dose limiting toxicity for CPT-11 is delayed diarrhea that occurs 48-96 hours following administration. This is thought to arise, in part, from direct activation of CPT-11, that is secreted in the bile, by a human intestinal carboxylesterase (CE) that is highly expressed in the small intestine. We hypothesize that by inhibiting this enzyme, reduced SN-38 will be produced from drug hydrolysis in the gut, thereby reducing the toxicity associated with CPT-11 treatment. The goals of this application are therefore to develop selective CE inhibitors that can be used to ameliorate the toxicity associated with CPT-11 administration. Highly potent, non-toxic small molecule inhibitors based upon the prototypical compound benzil have been identified, and we propose to develop these agents for use in inhibition of the human intestinal CE (hiCE). NMR, x-ray crystallography, medicinal chemistry, QSAR, biochemical and in vivo approaches will all be employed to validate the efficacy of suitable compounds The specific aims of this application are: 1) to determine the mechanism of CE inhibition by benzil; 2) to develop water-soluble analogues of benzil; 3) to assess the biochemical and biological properties of these compounds; and 4) assess their efficacy at modulating CPT-11-induced toxicity in a plasma esterase-deficient mouse model where hiCE is expressed in the mouse intestine. We believe that, if successful, these studies will provide reagents that can ameliorate the delayed diarrhea associated with CPT-11 administration, and potentially allow dose intensification of the drug. This would likely result in improved antitumor efficacy and furthermore, may also allow use of this CPT-11 against more resistant malignancies that demonstrate marginal response to this agent.